Exterior automotive trims made of resin, e.g., side moldings to be attached to exterior body sides, have hitherto been required not only to be less apt to suffer surface marring (for instance, from collision with objects) but to have other characteristics such as good dimensional stability and minimal thermal expansion. It is, however, difficult to form a side molding satisfying these requirements from a single resinous material. For example, a side molding made of a styrene-based thermoplastic elastomer alone has a high coefficient of linear expansion and therefore exhibits poor dimensional stability, although it is excellent in resisting marring. In contrast, the side moldings made of resinous materials having lower coefficients of linear expansion are inferior in marring resistance.
As an expedient for imparting low linear expansion to a sandwich molding as a whole while maintaining the marring resistance of the styrene-based thermoplastic elastomer, a side molding 21 as illustrated in FIGS. 4 and 5 has been proposed which has a sandwich structure consisting of a skin layer 22 and a core layer 23 (see, for example, JP-A-4-366635). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) In this side molding 21, the skin layer 22 is made of a styrene-based thermoplastic elastomer, while the core layer 23 is made of a mixture comprising a crystalline polypropylene resin having a low coefficient of linear expansion and an ethylene-.alpha.-olefin copolymer.
The side molding 21 having the structure described above is produced by a sandwich molding method. In this method, which is one mode of injection molding, a molten resin for forming a skin layer is first injected into a mold cavity and a molten resin for forming a core layer is injected shortly thereafter. Then, a small amount of the molten resin is again injected for skin layer formation. Thus, by this method the two kinds of molten resins packed into the cavity are cooled and solidified almost simultaneously.
Although the prior art technique described above can meet both of the requirements described hereinabove (e.g., marring resistance and dimensional stability), no measures have been taken against certain undesirable phenomenon occurring in the sandwich molding. For instance, the skin layer 22 sometimes develops linear sink marks (depressions) 24 at its surface parts corresponding to the side edges of the core layer 23. Sink marks 24 having a depth of less than 5 .mu.m, are not recognizable by the naked eye, and typically are not regarded as problematic. However, if the sink marks 24 have a depth of 5 .mu.m or larger, they may be seen by the naked eye and therefore impair the quality of appearance of the side molding 21.
It is thought that the generation of the sink marks 24 is attributable to the contraction upon cooling of the molten resin for skin layer formation and of the molten resin for core layer formation. That is, because crystalline polypropylene resin component has the largest shrinkage in the material for forming the core layer 23 and because it melts upon heating to usually 220.degree.-230.degree. C. and crystallizes when cooled to about 120.degree.-130.degree. C., molten resin containing crystalline polypropylene resin also shrinks considerably during cooling due to the shrinking of the crystalline polypropylene resin during crystallization. It is thought that this contraction of the molten resin occurs mainly in the width, as shown by the arrows in FIG. 5 (lateral direction in the drawing).
On the other hand, the molten resin for skin layer formation also shrinks during cooling. Since this contraction takes place along with the contraction of the molten resin for core layer formation as a matter of course, the two contractions affect each other.
If the shrinkage of the molten resin for skin layer formation and that of the molten resin for core layer formation are almost the same, (that is, if the shrinkage of the crystalline polypropylene resin in the layer 22 and the shrinking of the crystalline polypropylene resin in the layer 23 are almost the same), the sink marks 24 are not formed. Actually, however, the molten resin for core layer formation shrinks more than that for skin layer formation when the two molten resins cool and solidify in the mold cavity. In other words, the crystalline polypropylene resin in the core layer 23 shrinks more than that in the skin layer 22. Thus, it is assumed that an excessive stress is imposed on those parts of the surface of the skin layer 22 which correspond to the side edges of the core layer 23, so as to buffer the shrinking difference. As a result, the linear sink marks 24 are generated on the surface of the skin layer 22.
Illustratively stated, since the molten resin for skin layer formation has high flowability, it shrinks while being pulled in the arrow directions due to the contraction of the molten resin for core layer formation. It is assumed that an excessive stress is imposed, during this contraction, on those parts of the surface of the skin layer 22 which correspond to the side edges of the core layer 23 and, as a result, the visible sink marks 24 having a depth of 5 .mu.m or larger are generated on the surface of the skin layer 22.